The present invention relates to a relative position measuring apparatus using an optical displacement measuring device. More particularly, the invention relates to a high accuracy relative position measuring apparatus that employs a linear optical encoder system comprising a glass scale and a light detecting sensor.
Laser-based measuring apparatuses using lasers and optical encoder-based measuring apparatuses using optical encoders are known in the art. The laser-based measuring apparatus can achieve high measurement accuracy as measurements are made using the laser light wavelength as the unit of measurement. The laser-based measuring apparatus is primarily used as a relative position measuring apparatus for measuring the length between two points. The optical encoder-based measuring apparatus comprises: a scale constructed from a glass plate, film, thin metal plate, or the like; an optical grid formed with a prescribed pitch on the scale; a fixed index grid disposed opposite the scale with prescribed spacing provided therebetween (the phase of the index grid is 90 degrees shifted relative to the phase of the optical grid); a fixed light source for illuminating the scale with collimated light; and a light detection sensor. When the scale moves, the optical grid and the index grid overlap each other, producing a pattern of light and dark. The light detection sensor detects this light and dark pattern. The optical encoder-based measuring apparatus is commercially implemented as a digital gauge, and is primarily used as a relative position measuring apparatus for measuring the distance between two points. Optical encoder-based measuring apparatus according to the prior art will be described below with reference to relevant drawings.
FIG. 21 is a diagram showing a first prior art. The measuring apparatus shown in FIG. 21 comprises: a glass scale 10; an optical grid 11 formed on the glass scale 10; a light source 1 for illuminating the glass scale 10 with collimated slight; index grids 51 to 54 which receive light transmitted through the glass scale 10; an index base 50 where the index grids 51 to 54 are formed; light receiving elements 61 to 64 for receiving light transmitted through the index grids 51 to 54; and a substrate 20 where the light receiving elements 61 to 64 are formed. Also a semiconductor integrated circuit (IC) 22 and terminals 21 for connecting a cable 70 are formed on the substrate 20.
The phase of the index grid 51 is shifted by 90 degrees, the phase of the index grid 52 is shifted by 180 degrees, the phase of the index grid 53 is shifted by 270 degrees, and the phase of index grid 54 is shifted by 360 degrees relative to the phase of the optical grid 11. The light receiving elements 61 to 64 are each constructed from a signal light receiving element such as a photosensor.
The above-described first prior art is constructed by combining the glass scale, index grids, and light detection sensors, and thus the provision of index grids has been indispensable. Furthermore, to achieve high accuracy measurement, the pitch of the index grids, the proportions of the transparent and opaque portions of the index grids, the distance from the glass scale to the index grids, and the distance from the index grids to the light detection sensors must be adjusted accurately.
FIG. 22 is a diagram showing second prior art. The measuring apparatus shown in FIG. 22 comprises: a glass scale 10; an optical grid 11 formed on the glass scale 10; a light source for illuminating the glass scale 10 with collimated light; a light receiving array 37 for receiving light transmitted through the glass scale 10; and a substrate 20 where the light receiving array 37 is formed. Also a semiconductor integrated circuit (IC) 23 and terminals 21 for connecting a cable 70 are formed on the substrate 20.
FIG. 23 is a diagram showing the relationship between the optical grid 11 and the light receiving array 37 in the second prior art. Reference character s designates the pitch of the optical grid 11, while reference character w denotes the width of a transparent portion of the optical grid 11 and v denotes the width of an opaque portion of the optical grid 11. Here, w and v are each set equal to s/2.
The light receiving array 37 consists of a plurality of light receiving elements. Reference character p designates the pitch of the light receiving elements, while reference character u denotes the width of a light receiving portion 35 and r denotes the width of a light insensitive portion. Here, p=3/4xc3x97s, u=s/2, and r=s/4. That is, the ratio of u to r is 2:1.
More specifically, the light receiving array 37 is arranged so that four light receiving elements, g1, g2, g3 and g4, corresponds to three optical grid elements e1, e2 and e3. Further, the light receiving array is constructed so that every fourth light receiving element receives the same amount of light.
In addition, a light receiving element a1 is arranged so that its output is shifted in phase by 90 degrees relative to the output of b1, and a light receiving element b1 is arranged so that its output is shifted in phase by 90 degrees relative to the output of c1. Likewise, c1 and d1 are arranged so that their outputs are shifted in phase by 90 degrees, respectively. In this arrangement, as shown in FIG. 24, every four light receiving elements are connected together and their outputs are summed. Here, let the sum of a1, a2, a3, . . . be denoted by A, the sum of b1, b2, b3, . . . denoted by B, the sum of c1, c2, c3, . . . denoted by C, and the sum of d1, d2, d3, . . . denoted by D. Then, the phases of A, B, C, and D are shifted by 90 degrees relative to one another. The measuring apparatus makes a measurement by processing the signals of A, B, C, and D.
As described above, in the second prior art, the size of each light receiving element has had to be restricted to 3/4xc3x97s, and complicated wiring has had to be provided to enable data to be taken from every four elements in the plurality of light receiving elements and summed together.
Another prior art is described in Japanese Unexamined Patent Publication Nos. 8-313209 and 9-33210. This prior art uses a light detection sensor (CCD) having sensor elements arranged in an array at the same pitch as the optical grid, and the light detection sensor is constructed to also serve as an index grid. In this prior art, however, since the light source is moved together with the optical grid while holding the light detection sensor stationary, the length of the light detection sensor (CCD) must be made equal to the measuring length.
Still another prior art is described in Japanese Unexamined Patent Publication 10-132612. In this prior art, light detection sensors are arranged so that they are shifted by s/4 relative to one another. This prior art, however, has the problem that the pitch of the optical grid is coarse and the resolution is low, since four light detection sensors must be arranged within one pitch of the optical grid.
It is an object of the present invention to provide a compact and high accuracy optical displacement measuring apparatus that resolves the above-outlined problems.
It is another object of the present invention to provide an optical displacement measuring apparatus that uses a plurality of light receiving arrays each having a plurality of light receiving elements arranged at the same pitch as the optical grid.
It is still another object of the present invention to provide an optical displacement measuring apparatus capable of indicating the unit of measurement (1 xcexcm, 0.5 xcexcm, etc) using simple configuration.
The present invention comprises: a moveable first member having an optical grid formed with a pitch s; a light source for illuminating the first member; a plurality of light receiving arrays, each having a plurality of light receiving elements arranged at the pitch s, for receiving light transmitted through the first member; and a computing circuit for measuring a displacement of the first member based on outputs from the plurality of light receiving arrays, and wherein the plurality of light receiving arrays are arranged so that one is shifted from another by a predetermined distance in a direction of movement of the first member.
Preferably, the optical grid includes a transparent portion and an opaque portion, and the ratio of the width of the transparent portion to the width of the opaque portion is 1:1, while each of the light receiving elements includes a light receiving portion and a light insensitive portion, and the ratio of the width of the light receiving portion to the width of the light insensitive portion is 1:1.
Further preferably, the plurality of light receiving arrays are arranged along the direction of movement of the first member, or along a direction perpendicular to the direction of movement of the first member.
More preferably, the predetermined distance is equal to s/4 or a minimum measurement unit, for example, 1 xcexcm.
Preferably, the number of light receiving arrays is 2, 4, s, or s/2.
Further preferably, the light source emits collimated light.
In one preferred embodiment, the operating circuit includes: an intra-pitch relative position computing unit which performs a phase calculation based on the outputs from the plurality of light receiving arrays, and computes a relative position within one pitch from the phase calculation; a direction discriminating computing unit which discriminates the direction of movement of the first member based on the outputs from the plurality of light receiving arrays; a counter which counts a number of clear bands of contrast occurring due to the movement of the first member; a relative position computing unit which measures the displacement of the first member based on the result of the relative position computed by the intra-pitch relative position computing unit and on a count value supplied by the counter. Preferably, the optical displacement measuring apparatus comprises a display device for displaying the count value of the counter.
In another preferred embodiment, the computing circuit includes a converter which converts the outputs from the plurality of light receiving arrays into digital signals, a generator for generating a count signal for counting the amount of displacement of the first member based on the digital signals, and a counter which counts a leading edge or a trailing edge events of the count signal.
Preferably, an optical displacement measuring apparatus comprises a display device for displaying the count value of the counter.
More preferably, the count value of the counter corresponds a 1 xcexcm or 0.5 xcexcm displacement of the first member.
Since no index grid or the like is used, the present invention achieves an optical displacement measuring apparatus compact in size.
Furthermore, since the pitch of the light receiving elements is made the same as the pitch of the optical grid, and since the ratio of the width of the light receiving portion to the width of the light insensitive portion in each light receiving element is also made the same as the ratio of the width of the transparent portion to the width of the opaque portion in the optical grid, the invention achieves high accuracy with a simple configuration.
Moreover, in the optical displacement measuring apparatus of the invention, since provisions are made to generate a signal corresponding to a minimum measurement unit (1 xcexcm, 0.5 xcexcm, etc) and to display the measured result based on that signal, the measured result can be displayed using a simple and inexpensive configuration.